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of water dowsing-- finding underground water using forked sticks-- has been going on for centuries. typically, the dowser grasps the fork of the stick and points the other end skyward. then, as he's walking along and happens to pass over a supply of underground water, the end of the stick, or divining rod, supposedly twists downward, showing where to dig the well. in reality, almost anywhere you popoint the stick, you'd eventually find water. although its depth varies considerably from place to place, water is present beneath the earth's surface almost everywhere, even under the driest deserts. most people tend to take groundwater for granted, but it's a tremendously valuable resource upon which most of us depend. over 1/2 of the u.s. population relies on it for its drinking-water supply. even more groundwater's used for irrigating agriculture, and its industrial use is growing every day. groundwater is valuable because it's plentiful and clean. there's about 50 times more water underground than in all the lakes and rivers on the earth's surface combined. in many areas, especially
we call firn. it's an old swiss term that's still used today. so eventually the firn itself gets more compact, more recrystallized, and it becomes glacier ice. subject to extreme, instantaneous stress, ice shatters like glass. but if stress such as gravity is applied gradually over a long period of time, the ice bends. this process, called plastic deformation, explains how glaciers move. generally, ice must accumulate to a thickness of approximately 20 meters before movement starts. pulled by gravity, ice in a glacier typically shifts down slope a few millimeters per day. to study glacial flow, louis agassiz and his students built a hut on the ice itself. they observed that the center of the glacier moved most quickly, while friction slowed down movement along its sides. a similar phenomenon is observed in rivers and streams. scientists like agassiz also wanted to understand how glaciers flow internally. but it wasn't until early in the post-world war ii era that glaciologists were able to drill a hole through a swiss glacier. and this was a hole several hundred meters deep, maybe a c
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